1. Field of the Invention
This invention relates to an apparatus for suppressing vibrations or quaky movements in travel of a mobile or automotive type crane like rough terrain crane.
2. Prior Art
As illustrated in FIG. 5, mobile cranes generally have a boom 3 pivotally supported on a vehicle body 2 which is supported on driven vehicle wheels 4, for pivotal movements about a horizontal shaft 5 through a boom uplifting cylinder 4. With a mobile crane of this sort, it is usually experienced that the vehicle body 2 is subjected to vibrations and quaky movements during travel due to undulations or irregularities on road surfaces or due to abrupt accelerations or decelerations of the vehicle body 2, accompanied by vertical quaky movements of the boom 3 and other components which add to the vibrations of the vehicle body 2, giving ride discomfort to the operator on the vehicle.
For the purpose of damping such vibrations in travel, there has been known in the art an apparatus as disclosed in Japanese Laid-Open Patent Application No. 59-182195. As seen in FIG. 6, this prior art apparatus employs a damper mechanism 19 which is built in a boom uplifting hydraulic cylinder 18. It includes a counter-balancing valve 12 provided in a conduit 13 in communication with an oil chamber 181 which holds the load of the cylinder 18, and an electromagnetic change-over valve 16 and a shuttle valve 15 which are provided between the conduit 13 and a conduit 17 which is connected to the other oil chamber 182 or a conduit 14 which is in communication with a direction control valve 11.
According to this apparatus, if the direction control valve 11 is switched to a boom-up or boom-down position when the change-over valve 16 is in position A, the oil pressure is supplied to the oil chamber 181 or 182 of the cylinder 18 to stretch or contract the cylinder 18 for uplifting or lowering the boom. With a boom angle in excess of 10.degree., the change-over valve 16 is held in an interlocked state to prevent a switch to position B, and, with a boom angle smaller than 10.degree., the change-over valve 16 is relieved of the interlocked state. Thereafter, if a rocker switch is operated into an engaging position, the change-over valve 16 is switched to position B, communicating the conduit 13 with conduit 17 through the electromagnetic valve 16 and shuttle valve 15, thereby communicating the oil chambers 181 and 182 and an oil chamber 191 of the damper mechanism 19 with each other to form a closed circuit thereby suppressing vibrations relative to the vehicle body 1 when the vehicle is in travel.
In the above-described conventional apparatus, the boom angle alone is used as the criterion in making a judgement as to cancelling or holding the interlocked state. Namely, the interlocking is automatically cancelled as soon as the boom angle becomes smaller than 10.degree. during operation of the crane no matter whether a load is still in suspended state or the boom is in a stretched state. Under these circumstances, there are possibilities of the change-over valve being switched to position B due to an erroneous operation by the operator or for other reasons. Besides, after a vehicle travelling operation, the change-over valve 16 remains in position B if the boom angle is smaller than 10.degree., so that by mistake the operator might start a crane operation in this state by stretching or contracting the boom or hoisting a suspended load.
If the change-over valve 16 is switched to position B during a crane operation or if a crane operation is started while the change-over valve 16 still remains in position B after a vehicle travelling operation, the oil chamber 181 which holds the load of the cylinder 18 is communicated with the other oil chamber 182 by a closed circuit, and the pressure which acts in the oil chamber 181 for holding a load such as a suspended load also acts in the other oil chamber 182. Therefore, the effective pressure receiving area of the main piston 183 in the oil chamber 181 becomes an area which corresponds to the sectional area of the piston rod 184, resulting in an abnormal increase in the load holding pressure in the oil chamber 181. In some cases, depending upon the weight of the suspended load and the length of the boom 3, the load holding pressure in the oil chamber 181 could exceed the preset relieving pressure level of the counter-balancing valve 12 and open an overload valve of the counter-balancing valve 12 to contract the cylinder 18. Whereupon, there occurs a sudden downfall of the boom 3 and the suspended load, giving a great shock to the vehicle body 2. In the worst case, the cylinder 18 and the hollow piston rod (inner cylinder) 184 are damaged by overloading.
In addition to the above-mentioned problems, the conventional apparatus with the damper mechanism 19 built in the cylinder 18 is objectionably complicated in construction, and disadvantageous from the standpoint of fabrication process and cost. Moreover, since the oil chamber 191 of the damper mechanism 19 is constantly in communication with the oil chamber 182, it is put under the influence of pressure fluctuations in the oil chamber 182 even during crane operations and is therefore susceptible to troubles such as needing repairs and maintenance and service at an increased frequency.